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Police activity in Salisbury, England, March 7, 2018, around the home of former Russian double agent Sergei Skripal. The Russian ex-spy and his daughter were hospitalized after they were attacked Sunday with a nerve agent in a targeted murder attempt, British police said Wednesday. Sergei Skripal, 66, and his 33-year-old daughter Yulia were found unconscious on a bench in the southwest England city of Salisbury on Sunday.

A British police official on Thursday said that 19 people, in addition to the two previously reported, had sought medical treatment after a nerve agent attack on a former Russian spy.

The attack on Sergei Skripal, 66, and his daughter, Yulia, 33, is being treated as attempted murder, authorities have said. The two remain hospitalized in critical condition after being found unconscious Sunday on a bench near a shopping mall in Salisbury, 90 miles west of London, NBC News reported. A third person, a British police officer who tried to help the victims, also remained in the hospital Thursday, officials said.

Police have not offered any specifics about the attack, including the type of nerve agent used or how it was delivered.

Skripal, a former military intelligence officer, was sentenced to 13 years in prison in 2006 after being convicted in Russia of spying for Britain.